Is there a way to enter a precise amount of length or width for the measuring tool in Blender?
By displaying edge length in edit mode, you can view the edge length as you move a vertex. In 2.79 edge length can be enabled in the properties region N under Mesh Display, while in 2.80 it is in the viewport overlay menu in the header.

When you are moving a vertex, hold &#8679; Shift to slow down movement so that you can get the exact measurement you are after.

When you are trying to get the sum of multiple edges to equal a certain length, you can create a temporary edge and use snapping to align your vertex to the "measuring" vertex. Constraining movement to a single axis can also make this easier.